The present invention relates to a seat belt system that is provided on a vehicle, for example, an automobile, for the purpose of protecting an occupant. More particularly, the present invention relates to a seat belt retractor designed so that a reel shaft for winding up a webbing is locked from rotating when an emergency situation occurs, thereby preventing the webbing from being pulled out.
A typical conventional seat belt retractor mounted on a vehicle, for example, an automobile, is provided with a means for locking a reel shaft, which winds up a webbing, from rotating in order to prevent the webbing from being pulled out by the inertial movement of the occupant's body at the time of occurrence of such an emergency situation that a high degree of deceleration acts on the vehicle.
As one of the above-described reel shaft lock means, a frame lock type lock means is known in which a reel shaft and a frame, which supports the reel shaft, are formed with teeth, and when deceleration exceeding a predetermined value acts on the vehicle, the reel shaft is moved to engage the teeth thereof with the teeth of the frame, thereby locking the reel shaft from rotating, as disclosed, for example, in the specification and drawings of U.S. Pat. No. 4,796,918. In this frame lock type lock means, since teeth are formed on the frame, there is no need for a special member having such teeth, and it is therefore possible to form the lock means in a relatively lightweight structure. Accordingly, the above-described lock means makes it possible to comply well with the present demand for a reduction in weight of automobiles and other vehicles.
Incidentally, the conventional seat belt retractor, equipped with such a lock means, suffers from the following problems: When the teeth provided on the reel shaft engage with the teeth of the frame to lock the reel shaft from rotating, relatively large force is applied to each of the teeth of the reel shaft and the frame. Therefore, these teeth must be formed with an adequately large width in order to lessen the stress produced in the teeth. Measures which have heretofore been taken to lessen the stress produced in the teeth include one in which the thickness of the teeth of the reel shaft and the plate thickness of the frame are increased to thereby enlarge the width of area for meshing engagement between the teeth, and another in which the frame is provided with teeth, which are formed separately from the frame, thereby reinforcing the lock means. However, the former measure involves the problem that the weight increases, while the latter measure suffers from the problem that the number of parts required increases because of the need for a reinforcing material, and the number of working steps also increases because of the need for a step of attaching the reinforcing material, resulting in a rise in the cost.
Further, in the above-described frame lock type lock means, the teeth provided at both ends of the reel shaft must simultaneously mesh with the corresponding teeth provided at the left and right sides of the frame. In actual practice, however, it is difficult to allow these teeth to mesh with the corresponding teeth simultaneously. In many cases, meshing takes place only at one side of the frame. In particular, since the lock means is arranged such that the teeth on the reel shaft and the teeth on the frame are brought into engagement with each other by the movement of the reel shaft, it is extremely difficult to allow the teeth to simultaneously mesh with the corresponding teeth at both sides of the frame. If the teeth mesh with each other only at one side, the stress concentrates on the meshed teeth. Therefore, it is necessary to enhance the strength furthermore, and the retractor inevitably increases in the overall size and weight.